1. Field of the Invention
This invention relates to apparatus and processes for conversion of solar energy to chemical and electrical energy. Particularly, this invention provides for conversion of solar energy to electrical energy using a photoelectrochemical membrane cell to regenerate a redox anolyte of a redox-oxygen cell. An electron transferring membrane separates a first redox electrolyte comprising R/O couples from a redox aqueous anolyte comprising A.sup.+n /A.sup.+n-1 couples. Photosensitizers are located in the first redox electrolyte adjacent the electron transferring membrane and illumination of the photosensitizers produces excited sensitizers and electrons. The produced electrons are passed through the membrane oxidizing the sensitizers and reducing the redox anolyte, thereby producing chemical energy which, in the form of the reduced redox anolyte, may be stored for future energy utilization, including conversion to electrical energy. Particularly, the reduced redox aqueous anolyte may be converted to electrical energy in a redox-oxygen cell. The oxidized sensitizers may be regenerated by reduction in the first redox electrolyte and the first redox electrolyte regenerated electrochemically by contact with a negative electrode in communication through an external bias circuit with the redox aqueous anolyte.
2. Description of the Prior Art
The direct conversion of solar energy to electricity can be achieved by solid state photovoltaic cells, such as those using silicon solar arrays. While such direct conversion has been viable for specialized application, the costs have been too high for acceptance for general electrical generation utilization. Additionally, storage of electricity generated by the solid state photovoltaic cells has presented both technical and economic problems. The electricity storage has been generally performed by use of large storage batteries. Attempts are continuing to arrive at advanced battery systems which would provide technical and economic viability for such storage. Another method of conversion of solar energy to electrical energy in a single device is the photoelectrogenerative cell suggested by U.S. Pat. No. 4,037,029 wherein an electrolytic cell anolyte contains a photoelectrogenerative material.
The utilization of optical, or solar, energy to produce chemical energy has been recognized. For example, U.S. Pat. No. 4,094,751 teaches photoelectrolysis of water by utilization of photoactive photochemical diodes for decomposing water into H.sub.2 and O.sub.2. This patent teaches small photochemical diodes which are suspended in the bulk volume matrix of the chemical reactants and upon absorption of light, the diodes drive the desired chemical reaction within such matrix. However, the U.S. Pat. No. 4,094,751 does not teach utilization of photosensitizers in association with an electron transferring membrane to regenerate a redox anolyte and does not suggest regeneration of the sensitizers by utilization of an external bias circuit. U.S. Pat. No. 4,021,323 teaches the conversion of solar energy to electrical energy by using solid state photovoltaic generators to electrolyze an electrolyte thereby producing hydrogen for use in a fuel cell for production of electrical energy. The U.S. Pat. No. 4,021,323 does not suggest regeneration of a redox aqueous anolyte used in a redox-oxygen cell by utilization of photosensitizers in association with an electron transferring membrane.
In nature, the process of photosynthesis produces an oxidized species on one side and a reduced species on the other side of an energy transducing membrane in the chloroplast. Also, photo effects on chlorophyll containing bimolecular lipid membranes have been shown. (Tien, H. T., J. Phys. Chem. 72, 4512 [1968] and Tien, H. T. and Verma, S. P., Nature 227, 1232 [1970]) It is also known that artificial membranes doped with appropriate sensitizers and ionophores may generate photoelectric potential and current. (Tien, H. T. and Kobamoto, N., Nature 224, 1107 [1969]; Shieh, P. and Packer, L., Biochem. Biophys. Res. Comm., 71, 603 [1976]; Ullrich, H. M. and Kuhn, H., Biochim. Biophys. Acta 266, 584 [1972] and Pant, H. C. and Rosenberg, B., Photochem. Photobiol. 14, 1 [1971]) However, due to the large internal resistance of the membrane and the smallness of the produced currents, their use as current generators has not been practical. The formation of a donor/acceptor complex with a membrane by a sensitizer is known (Pant, H. C., et al supra) and the influence of the electron transfer process on the membrane due to an electrochemical potential gradient has been demonstrated. (Pant, H. C., et al supra) The cited prior art does not suggest regeneration of photosensitizers in a redox electrolyte which is regenerated by an external bias circuit to a redox aqueous anolyte.